DNA is made up of four types of molecules called nucleotides, designated as A, T, C and G. Base-pairing driven by hydrogen bonding, allows base pairs to stick together. Which base anneals to each base listed below?

Adenine(A) : Thymine(T)

Thymine(T) : Adenine(A)

Cytosine(C): Guanine(G)

Guanine(G) : Cytosine(C)

During which two steps of thermal cycling does base-pairing occur? Explain your answers.
Base-pairing occurs during the extended and final step of the PCR reaction. AT 72°C, the enzyme taq polymerase is at it's optimal temperature and proceeds to attach to primers on template DNA strands. Once attached, the enzyme attains the proper base pairs to create a complementary DNA strand for amplification.